The amount of information that can be stored on an optical record carrier depends, inter alia, on the size of the spot formed by the scanning device on the information layer of the record carrier. The information density and, hence, the amount of stored information can be increased by decreasing the size of the spot. The spot size can be reduced by increasing the numerical aperture of the radiation beam forming the spot.
An optical device of the above type is known from European patent application No. 0762398. An objective lens in the optical head of the device focuses a radiation beam from a radiation source to a spot on the record carrier. An adjustable diaphragm positioned between the radiation source and the objective lens adapts the numerical aperture of the radiation beam to the type of record carrier which is being scanned. The diaphragm is in the form of an electrically operated liquid crystal shutter. The shutter transmits only the central part of the incident radiation beam when scanning a first type of record carrier and transmits the entire radiation beam when scanning a second type of record carrier, thereby changing the numerical aperture of the beam incident on the record carrier. A disadvantage of the shutter is that it affects the wavefront quality of the transmitted radiation beam because of the different optical properties of the shutter in the central part and in the annular part around it. Another disadvantage is that the shutter constitutes an optical element which adds to the manufacturing cost of the scanning device.